Variable resistance materials are promising active materials for next-generation electronic storage and computing devices. A variable resistance material is a material that possesses two or more states that differ in electrical resistance. The material can be programmed back and forth between the states by providing energy to induce an internal chemical, electronic, or physical transformation of the material that manifests itself as a change in resistance of the material. The different resistance states are distinguishable and can be used as memory states to store or process data. Variable resistance materials offer the benefit of non-volatile performance.
Phase change materials are a promising class of variable resistance materials. A phase change material is a material that is capable of undergoing a transformation, preferably reversible, between two or more distinct structural states. The distinct structural states may be distinguished on the basis of, for example, crystal structure, atomic arrangement, order or disorder, fractional crystallinity, relative proportions of two or more different structural states, or a physical (e.g. electrical, optical, magnetic, mechanical) or chemical property. In a common embodiment, the two or more distinct structural states include differing proportions of crystalline phase regions and amorphous phase regions of the phase change material, where the phase-change material is reversibly transformable between the different states. In the crystalline state, the phase change material has lower resistivity; while in the amorphous state, it has higher resistivity. Continuous variations in resistivity over a wide range can be achieved through control of the relative proportions of crystalline phase regions and amorphous phase regions in a volume of phase-change material. Reversibility of the transformations between structural states permits reuse of the material over multiple cycles of operation.
Typically, a programmable resistance device is fabricated by placing the active variable resistance material, such as a phase change material, between two electrodes. Operation of the device is effected by providing an electrical signal between the two electrodes and across the active material. In a common application, phase-change materials may be used as the active material of a memory device, where distinct data values are associated with the different structural states and each data value corresponds to a distinct resistance of the phase-change material. The different structural states employed in memory operation may also be referred to herein as memory states or resistance states of the phase-change material. Write operations in a phase-change memory device, which may also be referred to herein as programming operations, apply electric pulses to the phase-change material to alter its structural state to a state having the resistance associated with the intended data value. Read operations are performed by providing current or voltage signals across the two electrodes to measure the resistance. The energy of the read signal is sufficiently low to prevent disturbance of the structural state of the phase-change material.
Phase-change memory devices are normally operated in binary mode. In binary mode, the memory is operated between two structural states. To improve read margin and minimize read error, the two structural states for binary operation are selected to provide a large resistance contrast. The range of resistance values of a phase-change material is bounded by a set state having a set resistance and a reset state having a reset resistance. The set state is a low resistance structural state whose electrical properties are primarily controlled by the crystalline portion of the phase-change material and the reset state is a high resistance structural state whose electrical properties are primarily controlled by the amorphous portion of the phase-change material. In addition to the resistance of the phase-change material proper, the measured resistances of the set and reset states of the device also include series resistances associated with the surrounding electrodes and elements. The set state and reset state are most commonly employed in binary operation and may be associated with the conventional binary “0” and “1” states.
In order to expand the commercial opportunities for phase-change memory, it is desirable to identify new phase-change compositions, device structures, and methods of programming that lead to improved performance. Many applications require memory that is stable at elevated temperatures. In the automotive field, for example, there is a need for memory that can perform in the high temperature environment at or near an engine. A current automotive design standard (AEC—Q100-005) calls for stable memory performance at a temperature of 150° C. for 1000 hours.
High thermal stability is also desirable in processes used in the manufacture of memory or components that include memory. In these processes, the memory material may be formed and/or programmed in a particular state and it is desirable to retain that state, without incurring the inconvenience of reprogramming, throughout backend processing or product integration that may require high temperature steps. As one example, a wafer containing memory is initially fabricated, probed and packaged, and the packaged parts subsequently need to be mounted on a printed circuit board. In a typical process, mounting is accomplished with a high temperature solder reflow process. Current solder reflow processes emphasize green lead-free solders that require exposing packaged parts to temperatures above 250° C. for ˜15 seconds.
Thermal stability is also important in archival memory applications. In these applications, information is stored in memory and is expected to be available for an extended period of time. Because of their non-volatility, phase-change materials are particular desirable for archival storage applications because memory states remain programmed without drawing power. In order to function effectively in archival applications, it is desirable for a memory material to retain its information for periods of several years or more at the extremes of climatic temperatures. If a memory material, for example, can store and retain data stably at temperatures of 50° C. (or higher to provide a margin of safety) for 10 years, it can provide archival storage without the need for air conditioning.
The thermal stability of phase-change materials is ultimately controlled by the tendency of the crystalline, amorphous, and mixed crystalline-amorphous structural states used as memory states to undergo thermal transformations. The extent to which memory states resist thermally-induced structural transformations at elevated temperature governs the suitability of phase-change memory for high temperature applications. Thermally-induced transformations of the relative proportions of amorphous and crystalline phases associated with a particular memory state have the effect of erasing or altering the memory state. As a result, the information initially programmed into a phase-change memory device is lost and the memory fails.
The data retention characteristics of prior art phase-change memory materials are inadequate to meet the needs of many high temperature applications. Known phase-change compositions are subject to rapid thermally-induced structural transformations when exposed to temperatures above 150° C. There is a need in the art for phase-change compositions that exhibit greater thermal stability.